1. Technical Field of the Invention
The present invention relates to an image forming apparatus such as printers, and more particularly, to an image forming apparatus wherein a plurality of image forming units are connected through their sheet transport units which are connected with each other, and a required number of image forming units are subsequently utilized for feeding a sheet in order to form images.
2. Description of Related Art
In recent years, there has been a growing demand for printers to be more sophisticated by adding such various functions as double-side printing function, composite printing function, color printing function and sorting function. On the other hand, a compact and inexpensive apparatus is also required though it is provided with only a single function, and a compact compact apparatus in low height has actually been introduced. However, for such an apparatus with a single function, there is not prepared any optional unit of the above-mentioned functions to be attached to the apparatus. The user who has been using a printer which is provided with a single function is, therefore, required to prepare a high-grade apparatus in place of the apparatus of a single function when it becomes necessary for the user to utilize the sophisticated functions. Since the high-grade apparatus is capable of fulfilling a function of the printer of single function, the printer with a single function which had been used becomes useless for the user. It will, therefore, be advantageous if the printer with a single function is used by adding the sophisticated functions to meet requirements.
For constructing a high-grade printer, it may advantageously be arranged to provide an inexpensive and versatile printer with various functions basing on the inexpensive printer with a single function.
In U.S. Pat. No. 4,972,236, there is disclosed an image forming apparatus wherein a plurality of image forming units are vertically stacked, and the paper feeding section and paper discharging section of an upper and lower image forming units are connected by a paper conveying unit provided with changeover means capable of changing over paper conveying path.
According to the image forming apparatus, double-side printing function, composite printing function, color printing function and the like may be provided by conveying sheets of copy paper in various modes with the paper conveying unit. A sorting function may also be provided by utilizing the paper conveying unit which is provided with a sorter section.
In the image forming apparatus disclosed in the official gazette, however, when a sheet is jammed in the paper feeding path of the image forming unit and in the paper conveying unit other than the uppermost image forming unit and the paper conveying unit or when various maintenance operations are conducted, it is necessary to clear the jam by opening upper frame of the image forming unit and opening upper cover of the paper conveying unit after each unit stacked therein has been removed which causes the operations more complicated.